A semiconductor device having thin film transistors (TFTs) is suitably used in an active matrix substrate of a display device. TFTs are classified into two types including those of a staggered structure (top-gate structure) and those of an inverse-staggered structure (bottom-gate structure). A polycrystalline silicon with which a high carrier mobility can be realized is mainly used in a semiconductor layer of a TFT of a staggered structure, whereas an amorphous silicon which can be easily formed through fewer steps is mainly used in a semiconductor layer of a TFT of an inverse-staggered structure.
In the display region of an active matrix substrate, pixel TFTs are provided as switching elements for pixels, and the OFF current of a pixel TFT is preferably small. In recent years, researches have been made for the provision of driver circuits, such as a gate driver for supplying scanning signals to gate bus lines and a source driver for supplying display signals to source bus lines, in the peripheral region which is provided in the periphery of the display region. In this case, the active matrix substrate includes not only the pixel TFTs provided in the display region, but also circuit TFTs provided in the driver circuit. The ON current of a circuit TFT is preferably high, and the circuit TFT is therefore designed so that the channel width thereof is large. However, if a high current flows through the channel region, heat may be generated in the semiconductor layer, thereby deteriorating the characteristics of the TFT or breaking the TFT. Therefore, researches have been made for TFTs with the aim of preventing the deterioration of characteristics due to heat (see, for example, Patent Documents 1 and 2).
A TFT disclosed in Patent Document 1 will now be described with reference to FIG. 9. FIG. 9(a) shows a top view of a TFT 800 disclosed in Patent Document 1, and FIG. 9(b) shows a cross-sectional view of the TFT 800.
The TFT 800 includes a gate electrode 810, a semiconductor layer 820, a source electrode 830, and a drain electrode 840. As can be seen from FIG. 9, the gate electrode 810 extends in parallel to the source electrode 830 and the drain electrode 840. The semiconductor layer 820 includes a semiconductor region 820a, and a semiconductor region 820b provided separately from the semiconductor region 820a. Note that in the TFT 800, a region of the semiconductor layer 820 that overlaps with the gate electrode 810 serves as the channel region. Heat generated in the semiconductor layer 820 is not only transmitted to the source electrode 830 and the drain electrode 840 via contact holes 835 and 845 but can also be transmitted to the outside from the boundary between the semiconductor region 820a and the semiconductor region 820b, thus suppressing the deterioration of characteristics due to heat.
A TFT disclosed in Patent Document 2 will be described with reference to FIG. 10. FIG. 10 shows a top view of a TFT 900 disclosed in Patent Document 2.
The TFT 900 includes a gate electrode 910, a semiconductor layer 920, a source electrode 930, and a drain electrode 940. Also in the TFT 900, a region of the semiconductor layer 920 that overlaps with the gate electrode 910 serves as the channel region. As can be seen from FIG. 10, a portion of the gate electrode 910 extends in parallel to the source electrode 930 and the drain electrode 940, but the gate electrode 910 is bent in a direction perpendicular to the direction in which the source electrode 930 and the drain electrode 940 extend in the region where it overlaps with the semiconductor layer 920. Thus, in the TFT 900, the gate electrode 910 is bent, thereby increasing the channel width with respect to the size of the semiconductor layer. Moreover, with the bending of the gate electrode 910, the electric field is reduced at least in areas where components of the channel region of different directions intersect with each other, thereby suppressing the heat generation. Therefore, with the TFT 900, the deterioration of characteristics is suppressed while the channel width is maintained to be relatively large.